1. Field of the Invention
This invention relates to a power converter and a power converter combined with a directional threshold detector to provide an AC-signal controller generally suitable for use in the two-wire portion of a telephone transmission line and particularly adapted for use in either a handicap-type telephone amplifier, a speakerphone amplifier, a repeater amplifier, or an echo suppressor.
2. Description of the Prior Art
Telephone subscribers with impaired hearing or weak speech require special purpose station apparatus which will provide quality voice transmission characteristics and compensation for their particular auditory or vocal disability. Consistent with the goal of providing universal service to all telephone subscribers, the operating company requirements are such that any special equipment furnished to a subscriber must be of a sufficient grade, quality and reliability not to degrade the existing service nor prove detrimental to the telephone network.
For purposes of this application the following terms have been defined as they are commonly understood in the telephone art.
The "two-wire" portion of a telephone transmission line is formed from a single two-wire cable pair. The "four-wire" portion of the telephone transmission line includes a pair of two-wire cable pairs (hence the term "four-wire"). The "hybrid network" (which typically includes an impedance matching network) connects the two-wire portion with the four-wire portion.
Voice signals in the two-wire portion of the telephone line are normally "bidirectional" since they may originate from either end of the line. The voice signals traveling on this type of line are superimposed if both parties are talking; normally the superimposed signals are separated into discrete signals by the hybrid network. The discrete signals can travel simultaneously in opposite directions in the four-wire portion without mixing because one voice signal path is provided for transmitting (i.e., "transmit channel") and another signal path is provided for receiving (i.e., "receive channel").
The prior art has provided auxiliary handicap amplifiers which are satisfactory for amplifying transmit or receive signals at the four-wire interface between the telephone hybrid network and the receiver wire pair or the transmitter wire pair. In each of the many types of existing telephone sets which may require a handicap amplifier, it is common practice for the hybrid network and thus the handicap amplifier to be located within the set proper, or the handset, as the case may be. This requires a number of custon designed amplifiers for the various applications. The prior art has not produced a universal four-wire interface handicap amplifier which is electrically and physically compatible with the different types of station apparatus, nor has it produced a satisfactory two-wire interface handicap amplifier which can be used external to, and independent of, the station apparatus.
One of the major problems facing any handicap amplifier designer is that of impedance mismatching between the auxiliary amplifier and the central office lines. Impedances vary from line-to-line due to different loop lengths. Impedance mismatching can seriously degrade the transmission performance of a line and can introduce unwanted signal reflections (i.e., "echoes"). Moreover, under certain closed-loop conditions, these echoes, if fed back to the input of the amplifier, could cause the amplifier to become unstable and oscillate (i.e., "singing"). Consideration of a similar oscillation or singing problem is also important in designing speakerphone systems, or two-way amplifiers (i.e., repeaters), or echo suppressors. It is elementary that the sum of the gains and losses around a closed-loop system must not exceed unity for stability.
Impedance matching is usually integrated within the hybrid network design to mitigate the impedance matching problems. An amplifier inserted in either wire pair of the four-wire interface need only provide an impedance match at one port and gain in one direction only. For this reason, the prior art amplifiers are almost invariably located in one or both channels of the four-wire portion of the telephone system. For many other reasons, however, it is desirable to locate the amplifier in the two-wire portion of the telephone line.
Two-way amplification in bidirectional two-wire transmission lines has been provided by negative impedance amplifiers, or hybrid-type amplifiers or voice-switched amplifiers. These types of prior art amplifiers have several inherent problems. Negative impedance amplifiers are highly dependent upon terminating impedances and must be carefully adjusted for stability. Moreover, since this type of amplifier cannot distinguish between either direction of signal transmission, reflected signals are amplified in the same manner as transmitted signals. Hybrid-type amplifiers require balancing networks which must be carefully adjusted to maintain satisfactory line balance characteristics in the presence of any changes in the electrical characteristics of the terminating network. Voice-switched amplifiers are commonly used to eliminate many of the adjustment problems inherent in the other types of amplifiers. However, there are other drawbacks to this approach. For example, voice-switched amplifiers amplify speech signals in one direction at a time, which usually means that the listening party can be heard only when the transmitting party chooses to stop talking.
The major difficulty with all of the abovementioned amplifiers is that when inserted into a two-wire network, their impedance characteristics become a part of the network. In the practical case of a two-wire cable pair, where impedance levels at the desired insertion point can vary, they can alter the existing impedance balance--thus requiring some form of impedance compensation for optimum performance.